[time-nuts] NTP Servers - RPi, NTP-Sec, Teensy 4.1, and one on the way from eBay

[Ben Hall]

Good evening!

On 1/10/2021 12:03 PM, ASSI wrote:
> All modern OS that NTPsec runs on provide nanosecond resolution.  The prcision
> and accuracy of those measurements vary with the hardware, the rasPi in
> particular is limited to something on the order of 150ns granularity as it
> runs the GPIO off a 19.2MHz clock.

Thank you much for your reply - I wasn't aware that the RPi ran the GPIO off of a 19.2 MHz clock, so this makes a lot of sense.  :)

> So, for a "normal" setup I'd trust maybe 10 microsecond figures, but you can
> get it down into the 100 nanosecond range with some care.

Very good!  Nice to know that the displayed figures are believable.  :)

>> delay   offset  jitter
>> =======================
>> 0.076    1.180   0.261
> 
> The jitter seems pretty high and the delay is oddly low.  Has the NTP loop
> actually converged already?  If you use a rasPi with Gigabit Ethernet make
> sure you switch off any link power management and set a fixed CPU frequency.

I'm not sure if the NTP loop has converged or not.  This RPi that I'm using has not had any power management or fixed CPU frequency adjustments - it has the stock software load.  However, now that it has been running for a while, ntpq is now reporting:

pi at spy-pi:~ $ ntpq -p
      remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
*192.168.4.66    .PPS.            1 u   94 1024  377    0.097    0.320   0.081
+192.168.4.50    .PPS.            1 u  849 1024  377    0.374    0.371   0.072
+192.168.4.57    .PPS.            1 u 1071 1024  377    0.341    0.377   0.114

So perhaps now the NTP loop has converged?  I'm not sure.

I should note:

192.168.4.66 is the Teensy 4.1 based GPS/NTP server.
192.168.4.50 is the RPi with standard NTP-based GPS/NTP server.
192.168.4.57 is the RPi with the NPT-Sec based GPS/NTP server.

Thanks much and 73,
ben, kd5byb